1. Field of the Invention
The present invention relates to an agent for enhancing a contrast, which is intended to clearly show an image of fluorescent staining of a lumen of the digestive tract with a fluorescent dye and raise diagnostic precision.
2. Description of the Related Art
In Europe and America, the onset of reflux esophagitis and Barrett's esophagus as gastroesophageal reflux-related diseases has increased in frequency in recent years. Various therapeutic methods have been developed for Barrett's esophagus, which is closely associated with esophageal adenocarcinoma. The epithelium of the mucosa that covers the internal surface of the esophageal wall is called the stratified squamous epithelium. This stratified squamous epithelium has many layers of cells and is evidently different from the form of the gastric mucosa, which has a simple columnar epithelium structure. Specifically, a site at which the forms of epithelial cells distinctly change is the boundary between the esophagus and the stomach.
Barrett's esophagus develops as a result of a prolonged course of reflux esophagitis caused by an inflammation in the esophageal mucosa due to gastric juice reflux into the esophagus. This disease is characterized in that the columnar epithelium of 3 cm or more in length is consecutively present all around the esophagogastric junction from the gastric side to the esophagus (Gastroenterology 92: 118-124, 1987).
There are only a few pathological reports on a mechanism though which the squamous epithelium is transformed into the columnar epithelium, and much remains to be elucidated. The epithelium within Barrett's mucosa having the morphological and histological features of the squamous and columnar epithelia is called the multilayered epithelium (ME). This epithelium is composed of four to eight layers of cells and is analogous in the deep part thereof to basal cells and in the surface thereof to the columnar epithelium. Furthermore, cytokeratin expression in ME has the natures of both the stratified squamous and columnar epithelia. A theory exists that this ME is a progenitor cell that gives rise to the columnar epithelium of Barrett's esophagus (Am J Surg Pathol 25: 569-578, 2001).
The early detection of a lesion is important for the diagnosis of the gastroesophageal reflux-related diseases. Therefore, the determination of progression of the diseases requires determining the degree of infiltration into tissues. This is because the early detection allows for not surgical resection but endoscopic mucosal resection. Patients who complain of a symptom are subjected to endoscopic observation, and their lesions can be detected easily if having distinct erosion or ulcer to the extent recognizable visually. By contrast, patients who have a symptom such as heartburn but no distinctly observed esophagitis are subjected to a method in which the esophageal pH is monitored for 24 hours. However, this method is psychologically painful for the patients.
The boundary between the stomach and the esophagus in cases of Barrett's esophagus is not easy to determine by usual endoscopic observation using white light sources. Therefore, random biopsy has been conducted to confirm the denaturation of mucosal tissues. However, an affected site or a region, even if diagnosed as cancer by biopsy, is difficult to be identified in many cases. Thus, dye-spraying endoscopy has been practiced, in which methylene blue staining has been used (Endoscopy 35: 998-1003, 2003). However, the methylene blue staining requires completely removing mucus, whereas staining using crystal violet clearly stains Barrett's mucosa but has no clear direct relationship with cancer detection (Stomach and Intestine, 39 (9), 1209-1210, 2004).
Magnifying endoscopes diagnose the depth of invasion by vascular network observation. However, changes in vascular network also differ depending on cancer differentiation and grades as well as on infiltration patterns. Therefore, a possible level of determination by the endoscopes is unknown. The use of the magnifying endoscopes has been said to exceedingly effectively conduct the qualitative diagnosis of cancer or noncancer and the diagnosis of the depth of invasion (Digestive Endoscopy, 14 (5), 565-572, 2002) and has been reported to improve diagnostic accuracy to 71% as compared with usual endoscopes and dye-spraying endoscopes, which have 45% diagnostic accuracy.
Confocal endoscopes, which has a usual endoscope equipped with a confocal imaging system, can produce a cross-sectional image of biological surface having intricately multilayered cells and connective tissues by detecting a biological tissue suspected of having a lesion by the usual endoscopic observation and observing the tissue by the confocal imaging system. This endoscope serves as means for performing noninvasive diagnosis on the basis of cell forms and tissue forms without collecting the tissues. To obtain the cross-sectional image, a fluorescent substance is usually administered to observed sites.
The fluorescent substance is a substance that emits fluorescence. A better fluorescent substance has less toxicity in vivo.
Thus, fluorescein has conventionally been used as a fluorescent contrast agent safe to living bodies in ophthalmoscopy or the like in which an aqueous solution of fluorescein is intravenously injected.
(Gastroenterology, 127 (3), 706-713, 2004)
However, studies conducted by the present inventor have demonstrated that the intravenous administration of fluorescein and the subsequent fluorescent observation of a lumen of the digestive tract do not produce clear fluorescent staining images in some cases. Further studies conducted by the present inventor have also demonstrated that such cases in which clear fluorescent staining images are not obtained are apparent especially for the upper digestive tract such as the stomach, intestinum duodenum, and esophagus.
Thus, an object of the present invention is to provide means for more clearly showing an image of fluorescent staining of a lumen of the digestive tract with an intravenously administered fluorescent dye.